The present invention relates to a method for fabricating a layer of semiconductor material having an arbitrarily preselected bandgap.
The present invention is applicable in general to any system where a semiconductor alloy is formed between two semiconductor materials having different bandgaps. (A semimetal material may be used in place of the narrow-bandgap semiconductor material.) The numerous known semiconductor species theoretically permit formation of many such alloy systems, such as Hg.sub.1-x Cd.sub.x Te or Al.sub.x Ga.sub.1-x As. However, it is frequently difficult to control the exact composition of an alloy which is formed between two such materials, and for some applications it is highly desirable to be able to control the composition of the final alloy achieved with some precision.
It would also be highly desirable to be able to produce a multi-color imager on a single monolithic substrate. At present, multi-color imaging is normally achieved by separately projecting images onto different image planes, with appropriate color filtering. However, a monolithic multi-color imager, if that were possible, would permit a much simpler arrangement.
In addition, further advantages may be provided by a multi-color detector which is separately sensitive to more than three wavelengths. While information at three separated wavelengths suffices to duplicate ordinary human color vision, independent sensing at a larger number of wavelengths may be desirable for certain diagnostic and analytic applications.
In addition, it may also be desirable to precisely select the bandgap of a semiconductor alloy for other purposes. See, e.g., Baliga, "Optimum Semiconductors for Power Field Effect Transistors", Electron Device Letter 162 (July 1981), which is hereby incorporated by reference.
Similarly, it would be highly desirable to be able to incorporate multiple light-emitting devices, operating at different frequencies in a single monolithic device. Such a structure would have useful applications both in multi-band optical communications systems, and also in displays.
One particular area where it would be desirable to be able to precisely select the bandgap of a semiconductor alloy is where the desired bandgap is extremely narrow. For example, for infrared detectors a material having a very small bandgap is desirable, so that infrared photons can excite carrier pairs, but the bandgap of the material must be non-zero, since a zero-bandgap region would be effectively shorted out.
Alloys of mercury, cadmium and tellurium are well known to be highly useful in fabricating infrared detectors. Since HgTe is a semimetal (having a very small negative band gap), and CdTe has a band gap of about 1.5 eV, compositions having an extremely small and arbitrarily selectable band gap may be specified simply by varying the proportions of an alloy having the composition Hg.sub.1-x Cd.sub.x Te. Such alloys are here referred to generically as "HgCdTe". For example, the composition Hg.sub..8 Cd.sub..2 Te is a 10 micron material, that is, a composition having a bandgap approximately equal to the photon energy of infrared light having a wavelength of 10 microns. By reducing the percentage of cadmium, compositions having a smaller band gap, and therefore a longer operating wavelength, may be produced.
Applications for such a semiconductor, having a small and arbitrarily selectable bandgap, are numerous. However, since the band gap varies with the composition, it is necessary for many applications that the composition of the alloy be uniform. In addition, it is of course necessary, for use in photodetectors, to provide materials which are relatively free of physical defects. Unfortunately, the characteristics of the HgCdTe system make preparation of such alloys difficult. In particular, it is highly desirable to provide an infrared detector operating at a wavelength of 12 microns or longer. Although HgCdTe alloys are transparent to wavelengths as long as 30 microns, and thus HgCdTe detectors operating at such wavelengths should in theory be possible, it has heretofore not been practicable to reliably fabricate HgCdTe alloys for operation at wavelengths significantly longer than 10 microns.
Heretofore long wavelength detectors have also been fabricated using doped semiconductors, such as silicon. With such material, the energy states provided by the dopants within the bandgap are used to provide a small transition energy, and therefore a long-wavelength absorption. However, intrinsic long-wavelength detectors are more efficient, and have much more definite frequency characteristics, than such doped materials. The present invention aims at providing an intrinsic long-wavelength detector, which has heretofore not been practicable to provide.
As discussed in U.S. Pat. No. 3,656,944, which is hereby incorporated by reference, a uniform mixture of mercury, cadmium and tellurium is usually achieved by preparing them as a homogenous liquid mixture. However, if such a mixture is cooled slowly, the solid which freezes out will have a progressively varying composition. To avoid these differential freezing effects, one method which has been attempted in the art is to quench a homogenous liquid mixture. However, two further difficulties arise in such a quenching process. First, as with most quenching processes where the solid state is significantly denser than the liquid state, the contraction of the liquid mixture as it solidifies is likely to cause formation of voids and "pipes" (that is, longitudinal voids near the center of a cylindrical body). Second, due to the very high vapor pressure of mercury at all temperatures of interest, it is difficult to prevent mercury from escaping from the solid-liquid mixture into any adjacent vacant space, including voids which may be created during the freezing of the mixture. U.S. Pat. No. 3,656,944 discusses ways to minimize this escape of mercury, but the method disclosed by this patent still permits significant inhomogeneity to remain in the alloy produced, and the imprecision of this method also does not permit full exploitation of the advantages which may be obtained, as discussed above, from selecting the band gap of the material produced by controlling the exact composition of the alloy used. Other methods of making HgCdTe have also not succeeded in attaining good yield rates.
Vapor phase epitaxy of HgCdTe has also been attempted, but this approach may result in a graded composition, and is believed not to provide the advantages of the present invention. See Becla, "A Modified Approach to Isothermal Growth of Ultrahigh Quality HgCdTe for Infrared Applications", forthcoming in J. Electrochemical Soc.
A further problem with present methods of HgCdTe production is that the area of the photodetector which can be produced is limited by the maximum single-crystal size which can be provided. Since the largest single-crystal size which is currently practical in production quantities is on the order of one inch square, this places a drastic size limitation on present HgCdTe detectors.
General references on the properties of CdTe and HgTe, and of certain other analogous ternary and quaternary systems, may be found in K. Zanio, 13 Semiconductors and Semimetals (1978), especially at pages 212 and following; and Harman, "Properties of Mercury Chalcogenides", in Physics and Chemistry of II-VI Compounds (ed. M. Aven & J. Prener, 1967); all of which are hereby incorporated by reference.
In addition, discussion of the properties and utilities of a wide variety of semiconductor alloys and compounds may be found in the following publications which are hereby incorporated by reference:
"Ternary Compounds, 1977", edited by G. D. Holah, Conference Series Number 35, The Institute of Physics, Bristol and London.
M. Gernard, "Glances at Ternary Compounds", Journal de Physique, 36, C3-1 (1975).
R. Nitsche, "Crystal Chemistry, Growth and Properties of Multi-Cation Chalcogenides", Journal de Physic, 36, C3-9 (1975).
R. C. Smith, "Device Applications of the Ternary Semiconducting Compounds", Journal de Physic, 36, C3-89 (1975).
It is also frequently desirable to be able to detect the spectrum of a distant object. One method for doing this is to image the same object on different detectors, each operating at different wavelengths. However, such a system requires precise optical calibration and adjustment, and, to resist decollimation, such a system must be made relatively bulky and heavy. Thus, it would be highly desirable to provide an infrared detector which could directly detect more than one wavelength on a single substrate.
It is an object of the present invention to provide compound semiconductor devices suitable for use as photodetectors. It is a further object of the present invention to provide HgCdTe films suitable for use as photodetectors.
It is a further object of the present invention to provide compound semiconductor devices, suitable for use as photodetectors, which have a very low density of material defects.
It is a further object of the present invention to provide compound semiconductor films, suitable for use as photodetectors, which have extremely homogenous composition.
It is a further object of the present invention to provide a method for producing compound semiconductor films wherein the exact composition of the final alloy may be accurately preselected.
It is a further object of the present invention to provide compound semiconductor devices, suitable for use as photodetectors, which have extremely flat surfaces.
It is a further object of the present invention to provide compound semiconductor films, suitable for use as photodetectors, which have a very large area. It is a particular object of the present invention to provide monolithic compound semiconductor films, suitable for use as photodetectors, which have an area significantly larger than one square inch.
It is a further object of the present invention to provide compound semiconductor films, suitable for use as photodetectors, which have an extremely low density of surface defects.
It is a further object of the present invention to provide a process for manufacturing compound semiconductor devices which provides an extremely high yield of satisfactory devices (i.e., number of satisfactory devices as a percentage of total devices).
It is a further object of the present invention to provide a method for manufacturing compound semiconductor devices, in which the yield rate is relatively insensitive to variation in parameters in the manufacturing process.
It is a further object of the present invention to provide a process for manufacturing compound semiconductor devices which does not require precise control of all manufacturing process parameters.
It is a further object of the present invention to provide a process for manufacturing compound semiconductor devices which includes self-limiting steps, so that the manufacturing process achieves no further effect on the device being manufactured, once the desired end product stage has been achieved.
It is a further object of the present invention to provide compound semiconductor devices for detection of very long wavelength light.
It is a further object of the present invention to provide compound semiconductor devices for detection of light at wavelengths longer than 12 microns.
It is a further object of the present invention to provide compound semiconductor structures having a uniform and extremely small non-zero band gap.
It is a further object of the present invention to provide a method for producing uniform films of an intrinsic semiconductor having an extremely small non-zero band gap.
It is a further object of the present invention to provide a method for producing monolithic compound semiconductor films wherein first portions have a band gap corresponding to a first wavelength and second portions have a band gap corresponding to a second wavelength.
It is a further object of the present invention to provide a monolithic compound semiconductor film for multi-color imaging.